A. The First Twister Separator
The closest prior art is the Twister Supersonic Separator (Trade mark, February 2001, F. T. Okimoto and M. Betting, Twister supersonic separator Proceedings of the 51st Laurance Reid Gas Conditioning Conference, Norman, Okla., USA). FIG. 1 is a schematic of the original Twister device. Its key feature is that a swirl-free flow goes through a converging-diverging nozzle and becomes supersonic. In this swirl free flow, water vapor condenses into droplets due to decrease in temperature. Then this supersonic flow becomes swirling due to an embedded wing.
Serious drawbacks of this Twister device are:    1. Droplets hit the wing with a high velocity and thus eventually destroy it. The Twister authors themselves pointed out that erosion of the wing is a problem.    2. Droplets have a very small time to reach the wall under the action of centrifugal forces because the length of swirling flow is short compared with its swirl-free part. Our estimate of the residence time shows that many droplets are not separated and go with the “dry” gas in the Twister device.    3. Twister does not permit a variable gas flow rate.    4. Significant (30%) pressure losses.
B. The 3S Separator
An alternative approach is the 3S (Supersonic Swirling Separation) technology developed by a group of Russian engineers (Vadim Alfyorov et al., Supersonic nozzle efficiently separates natural gas components, Oil & Gas Journal/May 23, 2005). FIG. 2 shows a schematic of the 3S device. In contrast to the original Twister device, here a flow first becomes swirling and then supersonic. This important feature is common for the 3S and the Sustor technologies.
The crucial difference is that the 3S technology uses a standard Laval (subsonic-to supersonic) nozzle, as FIG. 2 shows, while the SUSTOR technology uses a special nozzle which allows avoid the occurrence of vortex breakdown (VB). VB is typical of swirling flows and results in the appearance of a backward flow in a region downstream of the nozzle as FIG. 3 illustrates. This VB destroys the supersonic character of the flow in the working section due to a shock wave (the red curve in FIG. 3) developing upstream of VB and deteriorating the dehydration process. This limits the application of the 3S technology to weakly swirling flows (where VB does not occur). Since swirl is weak, the centrifugal effect is small and therefore the separation of liquid droplets (resulting from condensation in the supersonic flow) is inefficient. A significant share of droplets is not removed through the peripheral slit but is transported by the near-axis flow. These droplets evaporate as the flow becomes subsonic and its temperature recovers to its ambient value. Thus the dehydration is incomplete.